Microwave ovens use electromagnetic energy at frequencies that vibrate molecules within a food product to produce heat. The heat so generated warms or cooks the food. To achieve surface browning and crisping of the food a susceptor may be placed adjacent to the surface of the food. A typical susceptor comprises a lossy metallic layer on a paperboard substrate. When exposed to microwave energy the material of the susceptor is heated to a temperature sufficient to cause the food's surface to brown and crisp.
However, variations in the intensity and the directionality of the electromagnetic field energy form relatively hot and cold regions within the microwave oven. These hot and cold regions cause the food to warm or to cook unevenly. If a microwave susceptor material is present the browning and crisping effect is similarly uneven.
One expedient to counter these uneven effects is the use of a turntable. The turntable rotates a food product along a circular path within the oven. This action exposes the food to a more uniform level of electromagnetic energy. However, the averaging effect produced by the turntable's rotation occurs along circumferential paths within the oven and not along radial paths. Thus, even with the use of the turntable bands of uneven heating within the food are still created.
This effect may be more fully understood from the diagrammatic illustrations of FIGS. 1A and 1B.
FIG. 1A is a plan view of the interior of a microwave oven showing five regions (H1 through H5) of relatively high electric field intensity (“hot regions”) and two regions C1 and C2 of relatively low electric field intensity (“cold regions”). A food product F having any arbitrary shape is disposed on a susceptor S which, in turn, is placed on a turntable T. The susceptor S is suggested by the dotted circle while the turntable is represented by the bold solid-line circle. Three representative locations on the surface of the food product F are illustrated by points J, K, and L. The points J, K, and L are respectively located at radial positions P1, P2 and P3 of the turntable T. As the turntable T rotates each point follows a circular path through the oven, as indicated by the circular dashed lines.
As may be appreciated from FIG. 1A during one full revolution point J passes through a single hot region H. During the same revolution the point K passes through a single smaller hot region H5 and one cold region C1. The point L experiences three hot regions H2, H3 and H4 during the same rotation. Rotation of the turntable through one complete revolution thus exposes each of the points J, K, and L to a different total amount of electromagnetic energy. The difference in energy exposure at each of the three points during one full rotation is illustrated by the plot of FIG. 1B.
Owing to the number of hot regions encountered and cold regions avoided points J and L experience considerably more energy exposure than Point K. If the region of the food product in the vicinity of the path of point J is deemed fully cooked, then the region of the food product in the vicinity of the path of point L is likely to be overcooked or excessively browned (if a susceptor is present). On the other hand the region of the food product in the vicinity of the path of point K is likely to be undercooked.
Another expedient to counter the undesirable presence of hot and cold regions is to employ a field director structure, either alone or in combination with a susceptor.
The field director structure includes one or more vanes, each having an electrically conductive portion on a support of paperboard or other non-conductive material. The electrically conductive portions of the field director structure mitigate the effects of regions of relatively high and low electric field intensity within a microwave oven by redirecting and relocating these regions so that food warms and cooks more uniformly. When used with a susceptor the field director structure causes the food to brown more uniformly.
When an electrically conductive portion of a vane of the field director is placed in the vicinity of either an inherently lossy food product or a lossy layer of a susceptor attenuation of certain components of the electric field occurs. This attenuation effect is most pronounced when the distance between the electrically conductive portion of the field director and the lossy element (either the lossy food product or the lossy layer of the susceptor) is less than one-quarter (0.25) wavelength. For a typical microwave oven this distance is about three centimeters (3 cm). This effect is utilized by the prior art field director structure to redirect and relocate the regions of relatively high electric field intensity within a microwave oven.
FIG. 1C is a stylized plan view, generally similar to FIG. 1A, illustrating the effect of a vane V of a field director as it is carried by a turntable T in the direction of rotation shown by the arrow. The vane V is shown in outline form and its thickness is exaggerated for clarity of explanation.
Consider the situation at angular Position 1, where the vane V first encounters the hot region H2. Due to one corollary of Faraday's Law of Electromagnetism only an electric field vector having an attenuated intensity is permitted to exist in the segment of the hot region H2 overlaid by the vane V. However, even though only an attenuated field is permitted to exist the energy content of the electric field cannot merely disappear. Instead, the attenuating action in the region adjacent to the conductive portion of the vane manifests itself by causing the electric field energy to relocate from its original location A to a displaced location A′. This energy relocation is illustrated by the displacement arrow D.
As the rotational sweep carries the vane V to angular Position 2 a similar result obtains. The attenuating action of the vane V again permits only an attenuated field to exist in the region adjacent to the conductive portion of the vane. The energy in the electric field originally located at location B displaces to location B′, as suggested by the displacement arrow D′.
The overall effect of the point-by-point attenuating action produced by the passage of the vane V through the region H2 is the relocation of that region H2 to the position indicated by the reference character H2′. Similar energy relocations and redirections occur as the vane V sweeps through all of the regions H1 through H5 (FIG. 1A) of relatively high electric field intensity.
FIG. 1D is a plot showing total energy exposure for one full rotation of the turntable at each discrete point J, K and L. The corresponding waveform of the plot of FIG. 1B is superimposed in FIG. 1D as a dotted line thereover.
It is clear from FIG. 1D that the presence of a field director results in a total energy exposure that is substantially uniform. As a result warming and cooking of a food product placed on the field director will be improved over the situation extant in the earlier prior art. Similarly, the use of a field director in conjunction with a susceptor improves uniformity of browning of a food product.
The typical prior art field director is designed for minimum cost and is intended for a single (i.e., one-time) use for heating or browning a food product. When used in a microwave oven to heat a food product the field director structure warps and discolors due to the heat generated by the microwave energy. This problem is exacerbated when the field director is used with a susceptor. The warping and discoloration render the field director unsightly and may be of sufficient severity to render the field director unsuitable for a second use. Thus, the typical prior art field director is considered to be unsuitable for multiple uses.
In view of the foregoing it is believed advantageous to provide a field director structure that is both physically robust in construction and appropriately configured in arrangement so as to be able to withstand repetitive heating without loss of structural integrity. Such a field director structure could be advantageously used multiple times to heat a food product and, if used each time with a new susceptor, also to brown and crisp that food product.